Electrolytes for batteries and other uses generally need to be free of impurities that are detrimental to those uses. For example, in the context of a redox flow battery, each electrolyte used needs to be free of impurities that foul components of the battery. In a particular example, a vanadium redox flow battery (VRFB) is a system that converts electrical energy into chemical energy and then releases that chemical energy as electricity when there is demand. This type of battery is often paired with a solar and/or wind farm to help smooth out the power production intermittency associated with these renewable energy sources.
A VRFB comprises an electrochemical cell that performs the conversion between chemical and electrical energy. The electrochemical cell includes a negative electrode, an electrolyte separator (often a proton exchange membrane), and a negative electrode. Two separate vanadium solutions are stored in individual tanks—one tank contains a negative electrolyte solution that is fed to the negative electrode, and the other tank contains a positive electrolyte solution that is fed to the positive electrode. During normal operation, the negative electrolyte solution contains vanadium (II) and (III) ions, and the positive electrolyte solution contains vanadium (IV) and (V) ions. During charge, vanadium (III) ions are reduced to vanadium (II) ions in the negative electrolyte solution at the negative electrode, and vanadium (IV) ions are oxidized to vanadium (V) ions in the positive electrolyte solution at the positive electrode; the opposite happens during discharge.
When commissioning a new VRFB, a balanced electrolyte solution of average valence of roughly 3.5, i.e., an electrolyte solution of equal concentration of vanadium (III) and vanadium (IV) ions, is transferred into both the negative and positive electrolyte tanks. The battery is slowly charged until the negative and positive electrolyte solutions are at the desired ratio of vanadium (II)/(III) and vanadium (IV)/(V) ions, respectively. After this initial charge, some impurities present in the negative electrolyte solution will typically precipitate out as a solid metal phase precipitate. These impurities include, but are not limited to, As and Ge. These precipitates are detrimental to the electrochemical cell, because they clog the negative electrode and negatively impact battery performance.
The majority of conventional methods for making a vanadium-based electrolyte solution involve one of two methods:                Method 1: Mixing V2O3 and V2O5 in a 3:1 molar ratio in excess acid to produce a solution of a 3.5 average valence.        Method 2: Using a VRFB electrochemical cell in which the negative electrode is used to reduce a vanadium-based electrolyte solution to a 3.5 valence and the positive electrode is oxidizing a vanadium-based electrolyte solution that is periodically or continually reduced using a chemical reducing agent. This type of approach is needed because most organic reducing agents are only able to reduce vanadium (V) ions to vanadium (IV) ions (i.e. most organic reductants can't chemically reduce vanadium (IV) ions to a lower valence).        
Neither of these conventional methods adequately remove impurities that often foul the electrochemical cell of a VRFB. Rather, both methods require that the vanadium feedstocks have low impurity content of select impurities for proper function and inhibiting fouling in a VRFB system. Consequently, many purification methods focus on the purification of the vanadium feedstock. While many of these methods are effective, they consume many chemicals and the final product commands a high cost premium.